Epoxy resins are excellent in heat resistance, electrical characteristics, mechanical characteristics and adhesiveness so that the epoxy resins are used in wide fields such as fields of laminates, adhesives, coatings, molding materials and casting materials. In an electronic material field, in recent years, high frequency has come to be used in accordance with the progress of communication or computers, so that low dielectric characteristics are required for the purpose of increasing the speed of signal transmission. As methods for coping with the above requirement, a method which uses a dicyclopentadiene novolak type epoxy resin (for example, JP-A-11-060688) and a method which uses a biphenyl phenol aralkyl type epoxy resin (for example, JP-A-2002-179761) are known. In this respect, the present inventors have paid attention to polyphenylene ether resins which have properties of a low dielectric constant and a low dielectric loss tangent and developed a method in which an epoxy resin having a polyphenylene ether structure introduced therein is used (for example, JP-A-2003-292570). The epoxy resin having a polyphenylene ether structure introduced therein can have properties of high heat resistance, a low dielectric constant and a dielectric loss tangent, which polyphenylene ether resins have, but has a defect in that much time is required for curing, so that an improvement is required.
Conventionally, curable resins are widely used for adhesion, casting, coating, impregnation, lamination and molding compounds, etc. However, they are used in so various fields in recent years, and conventionally-known curable resins are sometimes dissatisfactory in some use environments or under some use conditions. For example, with regard to laminates for printed wiring boards used for various electrical devices, materials having low dielectric characteristics are required in accordance with the progress of electronic devices for the purpose of improving the speed of signal transmission. On the other hand, in recent years, materials are required to have higher heat resistance in addition to low dielectric characteristics under the influence of an increase of a reflow temperature due to the use of lead-free solder. Further, when used for electronic materials, curable resins are used in the form of varnishes in many cases. For this reason, it is required in view of workability that curable resins have excellent solvent solubility. As matrix resins which are recently used for laminates, phenol resins, epoxy resins, cyanate ester resins, vinyl ester resins and polyimide resins are known. These resins satisfy requirements such as high heat resistance and solvent solubility but do not sufficiently satisfy requirements of a low dielectric constant and a low dielectric loss tangent. As a curable resin which satisfies low dielectric characteristics, there are known a polyvinyl benzyl ether compound having a biphenyl phenol aralkyl structure introduced therein (for example, JP-A-2005-314556) and a bifunctional vinylbenzyl ether compound having a polyphenylene ether structure introduced therein, which the present inventors have developed, (for example, JP-A-2004-067727). However, further improvements in heat resistance and solvent solubility are required of these vinylbenzyl ether compounds. Development of a material having a balance of low dielectric characteristics, heat resistance and solvent solubility is desired.
Recently, the signal band of information communication devices such as PHS or portable telephones and the CPU clock time of computers reach to a GHz band and higher frequencies are coming to be used. The dielectric loss of electric signals is proportional to the product of the square root of dielectric constant of an insulator for forming a circuit, the dielectric loss tangent and the frequency of signals to be used. Therefore, the dielectric loss increases in accordance with an increase in the frequency of signals to be used. The dielectric loss damps electrical signals and impairs the reliability of the signals. For inhibiting the above impairment, it is required to select a material having a small dielectric constant and a small dielectric loss tangent as an insulator. As such a material, a fluorine-contained resin, a polyolefin resin, a polystyrene resin and a polyphenylene ether resin are proposed. However, though these resins are excellent in low dielectric characteristics, most of them have a problem about chemical resistance or moldability. With regard to films made of these resins for electrical insulating materials, for example, a film made of polyphenylene ether has a problem about flexibility (for example, JP-A-7-188362), and a film made of a low molecular weight styrene compound such as divinylbenzene is apt to have tackiness and a cured product obtained is fragile (for example, JP-A-2002-249531). The present inventors have proposed a resin composition having no tackiness and having low dielectric characteristics and high heat resistance by combining a vinyl compound derivative of a bifunctional polyphenylene ether oligomer with a high molecular weight compound (JP-A-2006-083364). However, since a solder reflow temperature is increased for coping with a recent demand for lead-free solder, materials which have further higher heat resistance and undergo only a small dimensional change under heat, are desired.